1. Field of the Invention
The present invention relates to night vision systems, and more particularly, to an improved night vision weapon sight.
2. Description of Related Art
Military and law enforcement personnel which use weapons, such as rifles, have long sought ways to improve their aim. By improving their shooting accuracy, these personnel increase their own effectiveness and survivability, while minimizing the possibility of innocent bystanders being inadvertently shot. The typical rifle is provided with a mechanical sight which is aligned to the barrel of the rifle. The operator visually aligns the mechanical sight with the desired target by peering down the barrel.
Since the desired target cannot be observed accurately at night, night vision systems are commonly used as night vision weapon sights. These systems employ an image intensification process which amplifies the ambient light reflected or emitted by an observed object. The image intensification process involves conversion of the received ambient light into electron patterns and projection of the electron patterns onto a phosphor screen for conversion of the electron patterns into light visible to the observer. This visible light is then viewed by the operator through a lens provided in an eyepiece of the night vision system. The night vision weapon sight is often equipped with a high power magnification, such as three times magnification (3X).
Instead of using the mechanical sight, internal sighting systems can be incorporated into the night vision sight. These sighting systems utilize a light source which is superimposed over the viewed image to provide an aim point, or reticle. The light source is aligned, or boresighted, to the barrel of the weapon, so that it designates the point which would be struck by a bullet fired from the weapon. The operator uses the night vision sight by overlaying the reticle over the image of the desired target viewed through the sight. Night vision sights having internal aiming reticles can enable an operator to accurately strike a distant target during low light conditions.
In typical night vision systems, an objective lens forms an inverted image on an internal image intensifier tube, which performs the image intensification process. In order for the system to present the user with an upright image, the image intensifier tube inverts the image. An inverting intensifier tube contains a fused fiber optic slug with a 180 degree twist. The eyepiece is non-inverting, and provides magnification of the image as desired by the user. Range focusing of the night vision weapon sight is accomplished by changing the relative distance between the objective lens and the image intensifier tube.
Non-inverting image intensifier tubes are used in other types of night vision systems, such as binoculars using a single image tube. For example, see U.S. Pat. No. 4,266,129, issued to Versteeg et al. However, non-inverting tubes have not heretofore been used in night vision sights since compatibility with traditional non-inverting eyepieces is desired. The least expensive and most uncomplicated eyepiece design is non-inverting; use of an inverting eyepiece causes the overall length of the sight to increase due to the addition of an inverting lens. Therefore, commercially available inverting image intensifier tubes were traditionally used in conjunction with non-inverting eyepieces in night vision weapon sight applications.
One problem experienced with such prior art night vision weapon sights is that of degraded aiming accuracy due to inadvertent motion of the inverting image intensifier tube. If the tube position is shifted laterally off-axis while the input image is held fixed, the output image will also shift laterally in the opposite direction. Slight lateral movement can unintentionally occur during focusing of the night vision sight, or by the mechanical shock of firing the weapon. Non-inverting tubes are insensitive to the lateral shift; if the tube position is shifted laterally while the input image is held fixed, the output image will remain fixed.
In all night vision aiming systems, it is desirable to have the aiming reticle be a contrasting color to the tube output image so that the reticle will be easy to distinguish from the scene. However, for the reticle to be a contrasting color, the reticle image must be inserted after the image intensifier tube and before the eyepiece. Therefore, any lateral movement of the tube output image will appear as a false apparent movement with respect to the reticle image. Since aiming reticles are often boresighted to a weapon with 0.1 milliradians accuracy, even slight false image movement of as little as 10 micrometers would invalidate the weapon boresight.
This problem could be alleviated by injecting the reticle image before the image intensifier tube. This way, the reticle and tube output image would move together. However, this approach diminishes the image contrast advantages described above. Moreover, the brightness of the reticle tends to "wash out" the scene images adjacent to the reticle, and can potentially even "burn" the reticle image permanently into the image intensifier tube. Thus, the disadvantages of this solution significantly outweigh the benefits.
Lateral movement of the image intensifier tube can be avoided by rigidly mounting the tube and reticle together, and move only the objective lens at the front end of the sight for focusing. Rotation of the lens causes it to move inward or outward relative to the image intensifier tube depending on the direction of rotation. Alternatively, a threaded ring could be provided which drives the lens without rotating the lens. A spring loaded rack and pinion or cam disposed on a shaft orthogonal to the optical axis could also be used. Nevertheless, these focusing methods often involve slight lateral shifts of the objective lens, which moves the scene image in the same manner as when the inverting image intensifier tube is shifted laterally. Thus, the same potential for aiming inaccuracy exists, and the problem has merely been transferred from the image intensifier tube to the objective lens.
An additional problem experienced with night vision sights which use image intensifier tube movement for range focusing is that of dioptric shift at the eyepiece. The eyepiece optics can be adjusted to accommodate the particular diopter of the operator's eye. Once the eyepiece has been properly adjusted, any movement of the focal plane of the image intensifier tube during range focusing would upset the diopter adjustment. To prevent the dioptric shift, the eyepiece can be movable in unison with the image intensifier tube. However, this renders the night vision weapon sight difficult to operate, since the operator would have to shift his eye position to accommodate each range change.
Furthermore, movement of the objective lens requires greater mechanical advantage than corresponding movement of the image intensifier tube. To allow a maximum amount of light into the sight, a large diameter objective lens is often utilized. Rotation of the lens can be cumbersome if the lens or threaded ring is large and awkward to grasp and rotate. Additionally, the lens must have sufficient internal friction so as to be intentionally difficult to rotate, and prevent unintended rotation out of adjustment due to axial shock experienced from firing the weapon. To rotate the lens, an operator must use a substantial amount of torque, requiring that a counter rotational force be applied to the instrument or to the weapon so as not to introduce cant to the alignment of the system. Besides, rotation of the lens is additionally undesirable since it could introduce unacceptable circular movement of the image viewed through the eyepiece of the sight.
A secondary problem also arises from the large size of the objective lens. The sight must be mounted on the weapon at a height sufficient to accommodate the lens size. As a result, the eyepiece of the sight would often be at a height which could be uncomfortable for the user. Weapons operators are trained to use their weapon with the mechanical sight provided with the weapon, which is typically mounted in close proximity to the top of the weapon barrel. These weapon operators practice firing their weapons with their head positioned to see the target and the mechanical sight concurrently. When the night vision sight is incorporated onto the weapon, the operators have to place their head in a different position to compensate for the higher eyepiece. Despite the inherent advantages of the night vision sight, many operators find them uncomfortable to use and less accurate because of the sight's awkward position.
This particular problem has been addressed in the prior art by U.S. Pat. No. 4,582,400, issued to Lough. The '400 patent discloses a night vision sight having an eyepiece disposed at an offset position which corresponds to the line of sight of the mechanical sight provided with the weapon. However, the Lough patent has a flaw which would render the design inoperable. In projecting the image to the eyepiece, the optical chain of the '400 patent repeatedly and unnecessarily inverts the image. The image is first inverted by the objective lens, and then inverted back to the upright configuration by the image intensifier tube. An additional lens in the eyepiece inverts the image once again, so that the final image presented to the operator remains inverted. Either the collimating lens after the image intensifier tube inverts the image as well, which adds unnecessary complexity to the scope, or the reference intends to present an inverted image to the operator. The '400 patent would be enabling if either a non-inverting eyepiece or a non-inverting image intensifier tube were used, although neither of these solutions were suggested in the reference.
Another problem experienced by users of night vision sights is that of calibrating the reticle. Ideally, the reticle should be positioned to precisely designate the target viewed through the night vision sight. However, the accuracy of the weapon can vary greatly due to external factors, such as windage and distance. A weapon operator is trained in making adjustments to the mechanical sight in azimuth and elevation to compensate for these external factors.
To adjust a reticle of a night vision sight, an operator physically moves a reticle light source along X and Y coordinates until a desired position is reached. This procedure can lead to inaccuracies since it is difficult to isolate the two measurements; often an adjustment to elevation results in inadvertent alteration to azimuth. The reticle position can also be influenced by shock or impact due to the weapon firing. As an additional problem, the reticle pattern may tend to rotate relative to the viewed image, which can disorient the operator.
Thus, it would be desirable to provide a night vision sight having an accurate and stable focusing system which does not require the operator to manipulate the objective lens. It would also be desirable to provide a night vision sight having improved aiming accuracy due to insensitivity to lateral movement of the image intensifier tube during focusing. It is additionally desired to provide a night vision sight having a simplified eyepiece which is offset to the line of sight of the mechanical sight typically provided with a weapon. It would also be desirable to provide a night vision sight which allows the reticle to be adjusted independently in azimuth and elevation.